1. Field of the Invention
This invention relates to a head drum servo control arrangement for video tape recording and reproducing apparatus, and is especially directed to a head drum servo control arrangement for adjusting head phase relative to the tape during an electronic editing operation.
2. Description of the Prior Art
During electronic editing of video tape, head phase errors can occur at the edit point or cut-in point; that is, a phase jump can occur when the phase of a video signal to be recorded does not match the phase of a video signal already recorded on the tape.
Although the format for recorded video signals on tape is standardized for helical scan video tape recorders or VTRs for broadcasting use, there is a certain amount of tolerance permitted. Consequently, from one VTR to another, there are small differences in the relation of the slant video tracks to control signals recorded longitudinally on an edge of the track, although these differences are all within a tolerance range. These differences appear as phase errors in the vertical synchronizing signal. Although these phase errors can be compensated during normal reproduction, certain problems arise during an electronic editing operation.
During electronic editing a new video signal is recorded, beginning at an edit point or a so-called cut-in point, immediately following a section in which a video signal has been recorded previously.
Because the vertical synch signals are used to control head rotational phase during reproduction, while position pulses generated by rotation of the head drum are used to control head rotational phase during recording, jumps in head phase can occur at the editing or cut-in points.
Consequently, in existing helical scan VTRs for broadcast use, means are provided to avoid generation of phase errors at editing points.
Generally, if a VTR is used in editing, first an editing point or cut-in point is determined. Then, the video tape is backed for a short distance to a point in advance of the cut-in point. After this, the tape is advanced and a reproducing operation is carried out until the cut-in point is reached. During a run-in period, that is during the time that the tape is advanced until the cut-in point is reached, the phase of the position pulse is displaced from a reference phase, and the phase difference between an off-tape vertical synch signal and a reproduced vertical synch signal separated from the reproduced video signal is adjusted until it is the same as the phase difference between the position pulse and the off-tape vertical synch signal. Thus, when the cut-in point is reached, the rotational phase of the head drum is properly adjusted to avoid phase jump at the cut-in point.
However, with an editing operation carried out as described above, jitter in the head drum prevents complete compensation of these phase errors. More specifically, if a sequence of short cuts (e.g., one to two minutes of video programming) are insert-edited onto a previously recorded tape using the above-described phase-correcting technique, phase errors will accumulate due to jitter of the head drum.